Photographic elements relying on silver halide emulsions for image recording have been recognized to possess outstanding sensitivity to light for more than a century. Roentgen discovered X-radiation by the inadvertent exposure of a silver halide photographic element. In 1913 the Eastman Kodak Company introduced its first product, a silver halide radiographic element, specifically intended to be exposed by X-radiation.
The utility of X-ray imaging as a medical diagnostic tool was immediately recognized, and the desirability of limiting patient exposure to X-radiation was also quickly appreciated. This led to the first X-ray imaging screens, specifically X-ray intensifying screens. These screens are constructed by coating a fluorescent layer on a support, usually a film support. The fluorescent layer is comprised of a mixture of phosphor particles and a binder. In use, an assembly is formed by mounting an intensifying screen with its fluorescent layer adjacent the silver halide emulsion layer of a radiographic element. An imagewise pattern of X-radiation striking the assembly is directly absorbed to a small degree by the silver halide emulsion layer. A much larger portion of the X-radiation is absorbed by the phosphor particles of the fluorescent layer. The phosphor particles promptly fluoresce at longer wavelengths which the silver halide emulsion layer can more readily absorb. A latent image is produced in the silver halide emulsion layer primarily attributable to fluorescence from the X-ray imaging screen. A summary of X-ray intensifying screens is found in Research Disclosure, Vol. 184, August 1979, Item 18431. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England.
Luckey U.S. Pat. No. 3,859,527 (reissued as U.S. Pat. No. Re. 31,847) proposed a second type of X-ray imaging screen, referred to as a storage phosphor screen to distinguish it from the X-ray intensifying screens described above. Storage phosphor screens can be essentially similar in construction to X-ray intensifying screens, differing primarily in the composition of the phosphor selected. Storage phosphor screens are imagewise exposed to X-radiation that is again absorbed by the phosphor particles. Although the phosphor may promptly fluoresce to some degree, most of the absorbed X-radiation energy is retained in the phosphor particles. When stimulated with longer wavelength radiation the screen emits in a third wavelength region of the spectrum. Typically X-ray imaging screens of the storage phosphor type are used alone for imaging--that is, these screens are not normally used to expose silver halide radiographic elements. Takahashi et al U.S. Pat. No. 4,926,047 is a recent example of the numerous patents that have sought to improve on Luckey.
Because of their structural and functional similarities X-ray imaging screens of both the intensifying screen and storage phosphor screen types encounter similar difficulties. The cost of X-ray imaging screens dictates that they be used repeatedly. To maximize their durability the screens are most commonly constructed using dimensionally stable polymeric films. Since the sharpest possible images are achieved with the thinnest possible fluorescent layer construction, the fluorescent layers are constructed with the minimum proportion of binder compatible with structural integrity--i.e., with a high weight ratio of phosphor particles to binder. To further protect the phosphor particles it is also conventional practice to coat a thin transparent film (commonly referred to as an overcoat) over the fluorescent layer.
In manufacturing scale construction a fluorescent layer containing phosphor particles and binder is coated on the planar coating surface of a continuous film as it is wound between storage rolls. To convert a wound roll of coated film into X-ray imaging screens the film is cut into convenient lengths. These cut lengths are then assembled into stacks, and the stacks are cut again to trim away edge areas, which are likely to contain coating irregularities. A third cutting step is usually undertaken to replace the corners with arcuate edges joining the perpendicular major edges. When cutting is undertaken by mechanical chopping, edge delamination (separation of the fluorescent layer form the film support) can occur.
When the X-ray imaging screens have been cut to size, the phosphor particles along the edges of the fluorescent layer are exposed. To protect the phosphor from degradation by exposure to contaminants it is common practice to seal the edges after cutting. The edge sealant can also be relied upon to physically protect the edges during handling in use. Thus, a series of cutting and sealing steps are conventionally employed to create an X-ray imaging screen from a fluorescent layer coated film roll.
A sectional detail of a conventional X-ray imaging screen 100 is shown in FIG. 1. A film support 101 is shown having a planar coating surface 103 bearing a fluorescent layer 105 which is in turn covered by a transparent protective overcoat 107. As shown the film support, fluorescent layer and overcoat have a common edge 109 produced by mechanical chopping. Flexing of the film support and fluorescent layer that occurs during mechanical chopping often results in areas of edge delamination, shown at 111. An edge sealant 113 applied in a post-chopping step is shown protecting the peripheral edge of the fluorescent layer that would otherwise be exposed.